A ball-nose end mill may be characterized by having, at its leading end, at least one effective cutting edge in the form of an arc of one quarter of a circle so that, upon the rotation of the end mill about its longitudinal axis, the cutting edge generates a hemisphere of a desired radius.
Depending on the cutting diameter of a ball-nose end mill, the effective cutting edge thereof can be obtained either by the use of a full-length single cutting insert with an arcuate cutting edge of 90.degree., which is typical for the tools with very small cutting diameters, or, in large tools, by the use of a plurality of partial length cutting inserts having overlapping arcuate cutting edges each being defined by an arcuate length less than 90.degree..
A small diameter ball-nose end mill with a single cutting insert is disclosed, for example, in Assignees' U.S. Pat. No. 5,622,460.
A large diameter ball-nose end mill comprising a plurality of cutting inserts mounted at a leading end of a tool holder is disclosed in EP 307 949. The cutting inserts are each rather small relative to the radial dimension of the tool holder and have relatively short arcuate cutting edges which overlap, when the inserts are mounted in the tool, whereby the possibility is provided for the use of the same inserts with tool holders with different cutting diameters and chip splitting is facilitated. Each cutting insert is of a generally triangular shape and has three identical indexable cutting edges. When such a cutting insert is mounted in a tool holder, one of its cutting edges is disposed in its operative position and the other two cutting edges define the insert body portion for mounting the insert in an insert receiving pocket. Consequently, the insert mounting space directly depends on the length of its cutting edges.
Whilst the above dependency may be perfectly tolerable in large tools, it is not suitable for smaller cutting tools where the size of a cutting insert is commensurate with the radial dimension of the tool. The use in such tools of cutting inserts requiring relatively large dimensions of insert receiving pockets, weakens the tool and limits the number the pockets which can be formed in the tool holder. Thus, known end mills of medium diameters usually employ only two inserts as a required minimum. An example of an end mill of this kind is disclosed in U.S. Pat. No. 4,252,480. The end mill comprises two indexable cutting inserts with cutting edges each of which has an arcuate a length of less than 90.degree., and which are mounted so that the cutting edges overlap to obtain an effective full-length cutting edge. However, the effectiveness of this end mill is not high.
To increase the effectiveness, U.S. Pat. No. 4,527,930 suggests the use of two identical non-indexable cutting inserts with full-length cutting edges, which are mounted symmetrically at a leading end portion of a tool holder. Each cutting insert comprises a substantially arcuate cutting head portion and a substantially trapezoidal mounting body portion integrally formed therewith. The cutting head portion has an extremely long cutting edge having a portion which defines an arc of 90.degree. and a straight portion continuous therewith. The mounting body portion has substantially planar upper and lower surfaces and substantially straight side walls converging in the direction away from the cutting head portion.
However, with the cutting edge of the insert described in U.S. Pat. No. 4,527,930 being very long, its mounting body portion still has substantial dimensions and, consequently, requires a relatively large insert receiving pocket. Moreover, the symmetrical arrangement of the inserts in the tool does not allow to effectively obtain center cutting without severely reducing the tool integrity.
It is therefore the object of the present invention to provide a new end mill and a new cutting insert for use therein, in which the above disadvantages are overcome.